1. Field of the Invention
The present invention relates to a variable magnification optical system used in a video camera, an electronic still camera or the like, and an imaging apparatus. In particular, the present invention relates to a variable magnification optical system that is especially appropriate for a surveillance camera, and which can be used both in a visible wavelength band and in a near-infrared wavelength band, and an imaging apparatus including the variable magnification optical system.
2. Description of the Related Art
Conventionally, a variable magnification optical system for CCTV (Closed-circuit Television) has been developed, as an optical system used in an imaging apparatus, such as a video camera, an electronic still camera and a surveillance camera, which uses an imaging device, such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor), as a recording medium. As such a variable magnification optical system for CCTV, a system consisting of four groups has many advantages, such as the simplicity of barrel-variable magnification mechanisms and the easiness of handling, and many optical systems of such a kind have been proposed.
For example, Japanese Patent No. 4454731 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2003-295055 (Patent Document 2) disclose four-group zoom lenses. The four-group zoom lens consists of a first lens group, a second lens group, a stop, a third lens group and a fourth lens group, which are in this order from the object side. The first lens group has positive refractive power, and is fixed during magnification change. The second lens group has negative refractive power, and moves in an optical axis direction during magnification change. The stop is fixed during magnification change. The third lens group has positive refractive power, and is fixed during magnification change. The fourth lens group has positive refractive power, and moves in an optical axis direction to focus during magnification change.